$5-90$ Consider a hollow$-$core printed circuit board $9$ cm high

and $18$ cm long, dissipating a total of $15$ W $.$ The width of the

air gap in the middle of the PCB is $0\mathrm{.}25$ cm $.$ If the cooling

air enters the $12-cm-$wide core at $25C$ and $1$ atm at a rate of

$0\mathrm{.}8\frac{L}{s},$ determine the average temperature at which the air

leaves the hollow core. Answer: $46\mathrm{.}0C$